1. Field of the Invention
The present invention relates to an improved fuse cutout and, more particularly, to an improved fuse cutout that has increased dropout characteristics and operating performance. The improved fuse cutout of the present invention is of the type shown in S&C Electric Co. Descriptive Bulletin 351-30, dated Dec. 7, 1998, entitled “S&C Type XS Fuse Cutouts” and in U.S. Pat. Nos.: 2,553,098; 2,745,923 and 4,414,527. This type of fuse cutout may be used with a fuse link of the type sold by S&C Electric Co. as the Positrol® Fuse Link and as generally shown in U.S. Pat. Nos. 4,317,099.
2. Discussion of the Prior Art
Fuse cutouts and fuse links utilized therein are well known. A typical fuse cutout includes a hollow insulative fuse tube having conductive ferrules mounted to the opposite ends thereof. One ferrule (often called the “exhaust” ferrule) is located at an exhaust end of the fuse tube and usually includes a trunnion which interfits with a trunnion pocket or hinge of a first contact assembly carried by one end of an insulator. The other ferrule is normally held and latched by a second contact assembly carried by the other end of the insulator so that the fuse tube is normally parallel to, but spaced from, the insulator. The insulator is mountable to the cross-arm of a utility pole or a similar structure. The fuse link is located within the fuse tube with its ends respectively electrically continuous with the ferrules. One point of an electrical circuit is connected to the first contact assembly, while another point of the circuit is connected to the second contact assembly. Often, the insulator and the fuse tube are oriented generally perpendicular to the ground so that the exhaust ferrule and the first contact assembly are located below the other ferrule and the second contact assembly. The fuse tube may include a high burst strength outer portion—for example, a fiber-glass-epoxy composite having an arc-extinguishing material within the inner portions thereof. Normal currents flowing through the electrical circuit flow without affecting the fuse link. Should a fault current or other overcurrent, to which the fuse link is designed to respond, occur in the circuit, the fuse link operates as described in more detail hereinafter.
Operation of the fuse link permits the upper ferrule to disengage itself from the upper contact assembly, whereupon the fuse tube rotates downwardly due to coaction of the trunnion and the hinge. If the fuse link operates properly, current in the circuit is interrupted and the rotation of the fuse tube gives a visual indication that the cutout has operated to protect the circuit, e.g. dropout operation to a so-called dropout position. Typical fuse links include a first terminal and a second terminal, between which there is normally connected a fusible element made of pure silver, silver-tin, or the like. Also connected between the terminals may be a strain wire, for a purpose described below. The second terminal is electrically continuous with, and is usually mechanically connected to, a button assembly, which is engagable by a portion of the upper ferrule on the fuse tube. The first terminal is connected to a flexible, stranded length of cable. Surrounding at least a portion of the second terminal, the fusible element, the strain wire (if used), the first terminal, and some portion of the flexible stranded cable is a sheath. The sheath is typically made of a so-called ablative arc-extinguishing material which, when exposed to the heat of a high-voltage arc, ablate to rapidly evolve large quantities of deionizing turbulent and cooling gases. Typically, the sheath is much shorter than the fuse tube and terminates short of the exhaust end of the fuse tube.
The free end of the stranded cable exits the fuse tube from the exhaust end thereof and has tension or pulling force maintained thereon by a spring-loaded flipper on the trunnion. The tension or pulling force exerted on the cable by the flipper attempts to pull the cable and the first terminal out of the sheath and out of the fuse tube. The force of the flipper is normally restrained by the strain wire, typical fusible elements not having sufficient mechanical strength to resist this tension or pulling force.
In the operation of typical cutouts, a fault current or other over-current results, first, in the melting or vaporization of the fusible element, followed by the melting or vaporization of the strain wire. Following such melting or vaporization, a high-voltage arc is established between the first and second terminals within the sheath and the flipper is now free to pull the cable and the first terminal out of the sheath and, ultimately, out of the fuse tube. As the arc forms, the arc-extinguishing materials of the sheath begin to ablate and high quantities of de-ionizing, turbulent and cooling gases are evolved. The movement of the first terminal under the action of the flipper, and the subsequent rapid movement thereof due to the evolved gases acting thereon as on a piston, results in elongation of the arc. The presence of the de-ionizing, turbulent and cooling gas, plus arc elongation, may, depending on the level of the fault current or other over-current, ultimately result in extinction of the arc and interruption of the current at a subsequent current zero. The loss of the tension on the stranded cable permits the trunnion to experience some initial movement relative to the exhaust ferrule which permits the upper ferrule to disengage itself from the upper contact assembly. This initiates a downward rotation of the fuse tube and its upper ferrule to a so-called “dropout” or “dropdown” position.
As noted above, arc elongation within the sheath and the action of the evolved gases may extinguish the arc. At very high fault current or over-current levels, however, arc elongation and the sheath may not, by themselves, be sufficient to achieve this end. Simply stated, at very high fault current levels, either the sheath may burst (because of the very high pressure of the evolved gas) or insufficient gas may be evolved therefrom to quench the high current level arc. For these reasons, the fuse tube is made of, or is lined with, ablative arc-extinguishing material. In the event the sheath bursts, the arc-extinguishing material of the fuse tube interacts with the arc, with gas evolved as a result thereof achieving arc extinction. If the sheath does not burst, the arc-extinguishing material of the fuse tube between the end of the sheath and the exhaust end of the fuse tube is nevertheless available for evolving gas, in addition to that evolved from the sheath. The joint action of the two quantities of evolved gas, together with arc elongation, extinguish the arc.
When a fuse tube is properly positioned between the upper and lower contact assemblies of the mounting, the contacts of the fuse tube are firmly engaged within the contact assemblies of the mounting. When the fuse link operates, gases evolved within the fuse tube thrust it against the upper contact assembly of the mounting. Ideally, the contact cap should not disengage the concavity until the fusible elements of the fuse link completely melts to release the tension in the cable and until the initial thrust of the fuse tube subsides. Release of this tension and subsiding of fuse tube thrust permits a limited amount of relative movement between the exhaust ferrule and the trunnion about a toggle joint therebetween. This limited movement permits the contact cap to move out of the concavity and the fuse tube to begin movement toward the dropout position due to rotation of the trunnion in the hinge pocket. If the fuse tube moves too far transversely during its thrusting, the contact cap may disengage the concavity too early. Third, transverse movement of the fuse tube can apply a bending movement thereon. This bending movement can fracture the fuse tube near the exhaust ferrule. Corrosion that builds up on various parts and dimensional changes of the fuse tube or fuse link sheath, e.g. due to environmental factors, can exacerbate the proper dropout action.
Thus, it is important for achieving proper operation as explained above that dropout operation be readily achieved in spite of any deleterious operating environments or conditions.